CCD sensors have excellent sensitivity at low lighting levels. They are for example capable of providing a usable signal for objects and scenes whose lighting levels may be as low as 0.003 lux.
However, if the objects or scenes to be observed are very bright, the elementary points or pixels of the CCD sensor receiving the most light are very rapidly saturated and a phenomenon is then observed which is known in the art by the English term “blooming”, which results from a greater exposure to light than that which takes a sensor pixel to saturation. The excess charges generated then interfere with the adjoining pixels. The result is thus a dazzling of the CCD sensor, and the image of the very bright object is unusable throughout the region adjoining this dazzling object. In the present description the term “blooming” will be used to designate this phenomenon of dazzling.
CMOS sensors are less sensitive to this phenomenon of blooming but cannot give a usable signal for objects or scenes which are not very bright.
It is well known how to attempt to reduce blooming by controlling the sensor or camera, reducing the exposure time of each image so as to reduce the blooming in very bright areas. The result then is that the contrast in the not very bright areas is greatly degraded. If the gain of the camera is then increased in order to improve the visibility of the low levels, the level of noise in the signal corresponding to the dark areas is also increased, which results in a “snowy” image of the dark areas, whose rendition is greatly degraded. Such a solution does not therefore make it possible to obtain a sufficient dynamic range for the signal, and hence it is for example desirable to obtain a usable image with a dynamic range of 170 dB between the signal corresponding to the darkest areas and the signal corresponding to the lightest areas of the same image.
Another solution is proposed by the document FR-A-2 565 753, which discloses a method for slaving the sensitivity of a photosensitive charge transfer device, used for example in a television camera, to the illumination which it receives, in order to avoid the blooming and saturation effects. This method consists of using antiblooming drains which the device has for discharging the charges generated by the light during a first part of the period of analysis of each frame. A slaving device determines the duration of the discharge according to the values of the video signal obtained. To discharge the charges to the antiblooming drains, which are kept at a constant potential by electrodes which surmount them, a pulsed voltage is applied simultaneously to the two transfer electrodes surmounting each photosensitive element of the device. A series of pulses is provided for discharging the accumulated charge fraction by fraction. This series of pulses is applied during the line suppression interval of the video signal in order to prevent stray couplings causing interference to this video signal.
Likewise, the document EP-A-1 237 363 discloses a solid-state image formation device comprising a solid-state image formation element supplying a plurality of image signals which differ from each other through their exposure time, and signal synthesis means for synthesising these image signals supplied by the solid-state image formation element, in which at least one of the image signals has a number of pixels smaller than the number of pixels of the other image signals, for the purpose of increasing the dynamic range of the retrieved images.
These last two solutions involve the production and use of specific photosensitive devices, which may not be adapted to all situations and which are relatively expensive.
There is also known from the document FR-A-2 660 822 a double-shot camera producing images with a high dynamic range, comprising a primary image sensor, a secondary source of image information corresponding to the same image as the one picked up by the primary image sensor but at a much lower sensitivity, and a means of combining the signals coming from the primary image sensor and the image information source.
All these known solutions are intended to increase the dynamic range of the final images supplied to the user but do not have as a common objective an increase in the sensitivity of the final image. Thus, where several images are captured of the same scene, the sensor supplying the one which has the highest sensitivity always remains in a dynamic operating range possibly going as far as saturation, but never as far as a phenomenon of blooming or dazzling.
The present invention is situated in this context and its purpose is to propose a method for the real-time processing of the signal generated by a sensor or a camera in order to supply, from a scene or object having a high dynamic range between the signal corresponding to the darkest areas and the signal corresponding to the brightest areas, a high-sensitivity image free from blooming areas and in which the areas whose luminosity is below a predetermined threshold are sufficiently contrasted, the predetermined threshold being able to be adjusted, for example according to the ambient luminosity or other criteria, such a method having to be able to be implemented without modification to existing sensors or cameras.